util/LazyRandomEngine: lazy wrapper for std::mt19937

Makefile.am

@@ -344,6 +344,7 @@ endif
 # Generic utility library
 
 libutil_a_SOURCES = \
+	src/util/LazyRandomEngine.cxx src/util/LazyRandomEngine.hxx \
 	src/util/SliceBuffer.hxx \
 	src/util/HugeAllocator.cxx src/util/HugeAllocator.hxx \
 	src/util/list.h \

src/util/LazyRandomEngine.cxx

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "LazyRandomEngine.hxx"
+
+void
+LazyRandomEngine::AutoCreate()
+{
+	if (engine != nullptr)
+		return;
+
+	std::random_device rd;
+	engine = new std::mt19937(rd());
+}

src/util/LazyRandomEngine.hxx

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_LAZY_RANDOM_ENGINE_HXX
+#define MPD_LAZY_RANDOM_ENGINE_HXX
+
+#include "check.h"
+
+#include <random>
+
+#include <assert.h>
+
+/**
+ * A random engine that will be created and seeded on demand.
+ */
+class LazyRandomEngine {
+	std::mt19937 *engine;
+
+public:
+	typedef std::mt19937::result_type result_type;
+
+	LazyRandomEngine():engine(nullptr) {}
+	~LazyRandomEngine() {
+		delete engine;
+	}
+
+	LazyRandomEngine(const LazyRandomEngine &other) = delete;
+	LazyRandomEngine &operator=(const LazyRandomEngine &other) = delete;
+
+	/**
+	 * Create and seed the real engine.  Call this before any
+	 * other method.
+	 */
+	void AutoCreate();
+
+	result_type min() const {
+		return engine->min();
+	}
+
+	result_type max() const {
+		return engine->max();
+	}
+
+	result_type operator()() {
+		assert(engine != nullptr);
+
+		return engine->operator()();
+	}
+};
+
+#endif